The present invention relates, generally, to a new and improved method for detecting the whirl of a drill bit, and/or the whirl of the Bottom Hole Assembly (BHA) in a drillstring used to drill oil and gas wells, and for reducing such whirl or whirls. As is well known in this art, "whirl" is used to describe the rotational motion of a bit, BHA or the drillstring itself, in which the bit, for example, is rotating at a different rotational velocity with respect to the borehole wall than it would be rotating if the bit axis were stationary. This precessional movement may be faster, or slower than the case where the bit axis is stationary. If faster, it is considered forward whirl; if slower, it is considered backward whirl.
Roller cone bits have been associated with axial vibrations since the first downhole measurements of forces and accelerations were first published. Measurements made while drilling with 3-cone bits have consistently and historically displayed axial vibrations at a frequency of 3 times the rotary speed, and when vibrations were severe the bit was observed to bounce. Cores have suggested that the vibrations are generated by a cammed bottom hole pattern, but it has not been determined whether this is the cause of the vibrations, or merely an effect.
The vibrations associated with polychrystalline diamond compact (PDC) bits are somewhat different than those of roller cone bits. Stick-slip torsional vibration of the drill string may be generated by dull PDC bits. PDC bits also vibrate laterally due to backward whirl. When this happens, the bit instantaneously rotates about some point other than the center of the borehole, and the point itself travels in a counter-clockwise direction around the borehole. Backward whirl has been identified as a primary contributor to the damage of PDC cutters, and simulation results suggest that its effects are amplified by torsional oscillations. Ways to mitigate this behavior have been investigated, and the most effective technique has become the basis for a popular commercial product line of PDC bits (anti-whirl bits), for example, as discussed in the SPE Paper No. 24614 entitled "Directional and Stability Characteristics of Anti-Whirl Bits With Non-Axisymmetric Loading", presented at the Annual Technical Conference and Exhibition, Washington, D.C., Oct. 4-7, 1992, by Pastusek, P. E., Cooley, C. H., Sinor, L. A. and Anderson, M.
Vibrations generated by the bit combine with those due to other sources, such as mass imbalance and wellbore friction, during drilling and reaming operations. The results are axial, lateral, and torsional vibrations of the drill string, which are believed to be a fundamental cause of drill string failures. Mathematical models have been developed by those in the art to identify and avoid operating parameters which lead to damaging downhole behaviors, but the complexity of the downhole environment limits the accuracy of model predictions.
In recent years modelling efforts have given way to monitoring efforts, as surface and downhole measurements have been used to identify harmful operating conditions. When sensors indicate that vibration levels have exceeded some safe level, the weight on bit and /or rotary speed are adjusted. If adjustments are not effective, and component failures are imminent, then the drill string must be pulled and its design modified.